1. Field of the Invention
This invention relates generally to appliances for medical use; and more particularly to a flush-valve assembly for a blood-pressure measurement catheter.
2. Prior Art
It is well known in the art of invasive blood-pressure measurement to determine a patient's blood pressure by monitoring the pressure in a liquid column that is in communication with the patient's blood stream. Thus in some cases a cannula is implanted in the patient's body, with its tip in a blood vessel. Typically a tube or "line" connects the cannula to an elevated supply container of saline solution or liquid used in medicating the patient. A blood-pressure measurement device is installed partway along the tube between the cannula and the supply.
In other cases, essentially the same system can be used with a special-purpose catheter that is implanted in a patient for other purposes. In particular, a cardiovascular catheter used for diagnosis or therapeutic purposes, or both, may have a lumen reserved for pressure measurement and/or administration of medication.
A slow flow, nominally three centimeters per hour, or as we will say herein a "drip-rate flow," of liquid into the patient's body is maintained to keep the cannula and supply line open, and to verify that it is open. Sometimes a small amount of anticoagulation compound, such as heparin, is added to the liquid.
Also within the range of "drip-rate flow," for neonatal, pediatric and some other special applications, are values such as thirty cubic centimeters per hour. In such applications administration of lipids or certain other medications sometimes requires larger orifices.
In starting and maintaining the operation of such a system, medical personnel must flush the blood-pressure measurement device and the line--to fill them, and to eliminate bubbles of air or any other gas. As is well known, removal of gas bubbles is important both for measurement accuracy and for patient safety.
By virtue of their compressibility, bubbles can support an unknown pressure differential between the measurement site and the patient's blood stream, directly constituting an error in the measurement. In addition, if bubbles are allowed to enter the patient's blood vessel serious injury to the patient can result.
Accordingly special flush-valve assemblies have been introduced to facilitate such flushing of the line and the measurement device, and to establish a suitable drip-rate flow after flushing is complete. U.S. Pat. No. 4,291,702, which issued Sept. 29, 1981, to Cole and Thornton, is representative of the state of the art in flush-valve assemblies for pressure-measuring catheters.
The Cole and Thornton device has a lever-actuated plunger that lifts an O-ring seal away from a seat to open the valve. When the valve is open it allows relatively high fluid volumes--or as we will here say in shorthand form, a "flush-rate flow"--of liquid to pass from the supply container through the pressure-measurement device to the blood-pressure measurement catheter. Usually flush-rate flows are manually variable, and typically perhaps two or three orders of magnitude higher than drip-rate flows.
The Cole and Thornton valve assembly also has a fine lumen or tube passing through the valve core, so that controlled drip-rate volumes of liquid can trickle from the intravenous-liquid or like supply into the catheter even when the lever is not actuated. Other patents, such as Schaberg and Cole U.S. Pat. No. 4,545,389, teach incorporation of a pressure-measurement sensor into a unitary assembly with the valve.
Such devices have been found extremely useful in medical applications and in fact enjoy wide commercial success Yet in some respects they bear improvement.
In particular, known valve assemblies are relatively costly. This is a particularly important limitation, since for many medical applications involving patients who may be carrying extremely dangerous and highly contagious disease, it is desirable to treat the flush-valve assemblies as disposable.
Furthermore, known valve assemblies have several regions where eddies tend to form in fluid flow, and also have some narrow niches or crevices between some components. Both these phenomena tend to trap gas bubbles in positions where they cannot be readily flushed out of valves during the routine preliminary flushing described above.
As already mentioned, bubbles can degrade measurement accuracy or even injure the patient. Generally a skilled user can remove all the bubbles, but only with difficulty and at the cost of some time.
Some commercially available devices have a thick-walled glass capillary tube that passes through an elastomeric tube. Drip-rate flow passes through the interior of the capillary.
On the outside of the capillary at about its midpoint is a circumferential boss or radial flange that stands the elastomeric tube away from the rest of the glass tube. To institute flush-rate flow, a user squeezes the outside of the device to deform the elastomeric tube.
In this deformation the elastomer is pulled away from the outside of the capillary, allowing fluid to flow through the resulting space. Such devices are described, in particular, by Young et al U.S. Pat. Nos. 4,192,303, 4,278,083, and 4,337,770.
These devices are useful but do have some drawbacks. In particular, it has been noticed that in some blood-pressure monitoring systems employing such valves there appear to be spurious oscillations in the blood-pressure readouts. These spurious oscillations interfere with the accurate and reliable measurement of actual variations in blood pressure.
Similar interferences have also been noted in some devices--e.g., the Cole-Thornton type--that use a spring-biased lever for flush control. In both cases, the interfering oscillations have been traced to mechanical resonances in the valves. Such resonances are sometimes within the effective frequency range of blood-pressure variations of interest, namely ten to fifteen hertz and below. Therefore these resonances mask or interfere with detection of these important variations in blood pressure.
The inventors of the squeeze-type valve, Young et al., in effect concede the presence of such problems in their earlier devices by including additional compensating structure in their two later continuation-in-part applications. In particular, they include within their resilient outer tube a rigid cylinder that radially constrains the downstream end of the resilient tube and to a large extent isolates it from the fluid column. In combination with other structural features, the rigid cylinder also constrains the inner thick-walled glass capillary tube both radially and axially.
Young et al. explain that the cylindrical extension and other constraining structures have "a beneficial effect on the wave forms and other clinical data produced by the monitoring apparatus." They do not, however, specifically teach either the problem cured by this "beneficial effect" or indeed what the effect itself is--even though it is apparently the subject of a continuation-in-part application and a divisional therefrom.
It seems likely that the "beneficial effect" is some reduction in resonant interferences. Even the newer configuration disclosed in these two later patents, however, is not completely free from those interferences.
The Young configurations also have additional important limitations, common in other valves commercially supplied for blood-pressure work. First, they have highly objectionable crevices and backwater regions where bubbles can be trapped and resist flushing. Prime among these regions, for instance, is the very long annular space between the new rigid cylinder and the resilient outer tube, in their newer configuration. This space would appear to be very difficult to debubble completely.
Secondly, the Young devices present a "funnel" effect at the entrances to their fine capillary drip-rate bores. Such geometry is extremely susceptible to occlusion of the capillary bore by fluid-borne particles carried in the intravenous liquid. It will be appreciated that even microscopic particles can completely clog a capillary bore that is only a few hundredths of a millimeter in diameter.
Thirdly, some users find it difficult to smoothly control the outer-tube squeeze action used for flush-rate control. This difficulty may be partially a matter of the size of the apparatus relative to the size of the user's hand, or in some cases partially a matter of manual strength or dexterity, but in any event a smoother progressive control of flush-rate fluid flow is desirable.
At the same time, there are commercially available devices that have a snap-back action, for use when flushing is complete, that is more positive than the outer-tube squeeze action in resealing the flush path and thus returning to the drip-rate flow. Some users feel that this more-positive snap-back action is preferable to the other types of return action. It would be ideal if a user could simply choose between smoother progressive control or snap-back action, as preferred at the time of each use.
One device that has a snap-back action is disclosed in Reynolds and Sorenson U.S. Pat. No. 3,675,891. That document may be regarded as the seminal patent in resilient-core valves for blood pressure monitoring systems.
The valve has an elastomeric core in a rigid valve body, and a drip-rate bypass that is embedded in the valve body. Partly because the bypass is associated with the body rather than the core, both the drip and flush flows follow dogleg paths, resulting in some erraticism of debubbling as will be seen, and also some additional cost.
The valve-core opening and closing action is longitudinal with respect to the flush-rate fluid flow--that is to say, the valve core moves bodily parallel to the flush flow, riding on a long cylindrical bellows that lies within the flush channel and is unitary with the core (but functionally distinct). During drip flow, the long bellows is not exposed to the measurement fluid column directly, but only through the seated valve core.
The valve core is conical and engages a conical seat at the downstream end of the bellows. The seating of the core is thus along a thin annular area encircling the center of the conical core. To flush the system, a user pulls outward on a central stem that is attached at the back of the core portion and that extends outward through the long bellows to the exterior of the valve.
Since the core itself is elastomeric and thus resilient, it requires no separate O-ring or like seal to seat hermetically. Further, since the core and bellows are formed as a unitary part, the resilience of the elastomer also provides the necessary biasing action of the cylindrical bellows.
Nevertheless it is noteworthy that, in opening and closing of this valve, the major component of motion is bodily displacement of the entire core portion, as distinguished from resilient deformation to open some part of the valve/seat interface. In this way the underlying operating principle of the Reynolds-Sorenson device is substantially the same as the Cole-Thornton unit.
The core center itself is never restrained but rather extends as a free-floating guide tip into a narrow flush-path outlet chamber. This tip helps the core to reseat reliably when released after flushing.
In the Reynolds-Sorenson configuration both the flush valve and the drip bypass follow dogleg routes, making the likelihood of bubble entrapment strongly dependent upon the orientation of the valve. Debubbling is accordingly tricky.
In particular, bubbles may be too easily trapped below the bellows 28, 31 (as drawn, in Reynolds' FIGS. 2 and 5 respectively), at the bottom right-hand corner of the flush channel 19; or in the top channel 14. During drip flow, bubbles also may be trapped in the flush path outlet chamber 20 and the adjacent lower corner of the cross-connect path 17.
The Reynolds-Sorenson valve is also subject to interfering-resonance problems of the type discussed above. Although in this valve such problems are less severe than in the Young valves, they are nevertheless significant.
In addition, flushing the Reynolds-Sorenson valve requires either considerable dexterity or the use of two hands. Reliable snap-back reseating is provided, but the manual pull-stem does not lend itself to smooth control of flush rate. Finally, because of "funneling" as mentioned above in relation to the Young patents, the Reynolds-Sorenson drip bypass is difficult to keep clear.
It is known among designers of flush-type valves that the relatively low-frequency mechanical resonances found to intrude into the measurement system from some prior-art valves can be understood by analyzing the valve structure, the catheter and other tubing, and the pressure monitor all considered together as a resonant mechanical system. Despite this general understanding, however, prior artisans have failed to reduce such resonances to negligible magnitudes.
Such analysis reveals that relatively low-frequency resonances can arise from a relatively high degree of mechanical compliance or resiliency of some components of a valve unit, as "seen" by the fluid columns in the valve unit and in the fluid-supply lines.
It is well known that in any mechanical system which has a compliant or energy-storing component, resonances are possible at frequencies which vary in an inverse way with the amount of compliance or resiliency. Hence resonance in a relatively low range of frequencies--such as zero to ten or fifteen hertz, the effective frequency range of interest for blood-pressure measurements--can occur if a resilient component or subunit of the valve is too resilient.
It follows that the generation of interfering resonances in this range of interest can be significantly reduced by using less-resilient (i.e., stiffer) materials wherever resilience is required. Interference can be further minimized by reducing the gross size of the resilient or energy-storing element.
That is to say, the magnitude of a resonant vibration can be lowered by decreasing the amount of energy that can be stored in the resilient element. This can be done by reducing the mass of that element.
Still further, the practical effect of a resilient or energy-storing element in a mechanical system can be minimized by lowering the coupling between the resilient element and the rest of the system. That is, if a particular resilient element is present but can neither transfer energy to nor receive energy from the rest of the system efficiently, then the system behaves as if the resilient element were smaller.
These considerations favor valve configurations having very little resilient surface in contact with the pressure-transmitting fluid--or, stated more generally, these principles favor having very little surface that transmits forces between the pressure-transmitting fluid and a resilient element of the system.
Although these principles are known, prior-art valves are subject to objectionable levels of mechanical resonance. Heretofore no valve configuration has been found that makes optimum use of these principles--at least not without compromises that introduce other operating problems.
For example, analyzing the previously discussed Cole and Thornton valve unit, it can be seen that the apparatus is possibly subject to undesirable mechanical resonances because of the mechanical compliance present in the subunit consisting of the valve core, lever and plunger, and a resilient spring that biases the core seal (i.e., the O-ring) against its seat. This composite structure or subunit is directly in contact with the measurement-fluid column, over the face of the valve core--a relatively large surface area (very roughly twenty square millimeters) for transmission of fluid pressure.
Similarly analyzing the Young devices, in the original design the downstream half of the elastomeric outer tubing was in direct contact with the measurement fluid column. This contact extended entirely around the internal circumference of the tube, a much larger surface (very roughly one hundred sixty square millimeters) than in the Cole-Thornton valve; and the elastomeric tubing was loosely in tension, tending to exaggerate or at least not minimize its resiliency.
In the later Young devices, the interposed rigid cylinder reduced the mechanical coupling of the downstream fluid column with the elastomer very greatly, but not entirely--since a narrow annular fluid column remains.
Similarly reviewing the Reynolds-Sorenson unit, it can now be appreciated that the fundamental geometry remains the same as in the Cole-Thornton unit. This is so despite the relatively sophisticated integral design of the valve core, face, seat, biasing bellows and actuator as a single molded elastomeric subunit.
Each unit has a bodily moved core that is exposed across its face area to liquid pressure, and that couples a large compliance to the liquid through that face. In the Reynolds-Sorenson valve this surface (roughly ten or fifteen square millimeters) appears somewhat smaller than that in the Cole-Thornton valve, but remains significant.
In addition another transmission mechanism may operate to couple compliance to the measurement liquid column. This is a second route via the central "core of the core."
By that we mean the central part of the valve core, within the annular seat area, where the elastomeric material is only partially compressed and therefore somewhat resilient and somewhat free to vibrate. A significant surface area of this downstream end of the conical core and of the guide tip is exposed to the measurement fluid in the outlet chamber of the flush valve.
To the extent that vibrations may possibly be transmitted through the core, the long, thin-walled bellows is coupled to the fluid column. Even considered alone the bellows has a sizable resilient surface area and mass.
From this presentation it will be understood that the prior art has not entirely satisfied the need of medical practitioners for an inexpensive flush-valve assembly that has little or no tendency to trap bubbles; that can be used in a smooth progressive-control mode for flushing or in a positive snap-back mode for restoring "drip" operation, as preferred; and that can be made in such a way as to avoid resonances that mask pressure variations.